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Giardia infection, stool testing, metronidazole and knowing when it is more than a stomach bug.

The most common intestinal parasite in the UK - and one of the most curable. A clear diagnosis and a short course of treatment usually settles the whole thing.

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Why trust this guide

  • 01

    Editorial team

    Written by our editorial team from the UK sources listed on each page.

  • 02

    Sourced from guidance

    Checked against UKHSA, NICE CKS and Hospital for Tropical Diseases guidance you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK guidance including stool PCR panels, metronidazole first-line and nitazoxanide alternatives.

Key facts

Giardia infection at a glance.

The essentials, in plain English - what it is, how it spreads, and how it is treated in the UK today.

  • What it is

    Protozoal infection of the small intestine caused by Giardia duodenalis (also called G. lamblia or G. intestinalis) - the most common intestinal parasite in the UK.

  • How you catch it

    Faecal-oral transmission - contaminated water (a big one for hikers, travellers, wells and unfiltered surface water), person-to-person in nurseries, food and animals such as dogs and beavers.

  • Incubation

    Symptoms usually appear one to three weeks after exposure - long enough that people often forget the trigger.

  • Classic picture

    Watery diarrhoea, foul-smelling steatorrhoea, cramps, bloating, flatulence, weight loss, nausea and fatigue - often self-limiting over two to four weeks.

  • Chronic form

    A relapsing course with malabsorption is well described - in children it can present as failure to thrive.

  • First-line therapy

    Oral metronidazole 400 mg three times a day for five to seven days - tinidazole 2 g as a single dose is a common alternative.

Why this guide matters

A treatable infection - not a mystery bug.

Giardia is often missed because the incubation is long and the symptoms overlap with common gut complaints. Three ideas shape everything else on this page.

  • Stool testing changes things

    A giardia antigen ELISA or PCR panel confirms the diagnosis quickly - so treatment is targeted rather than guessed.

  • Short courses cure most people

    Metronidazole for five to seven days, or a single dose of tinidazole, clears the parasite in the majority of cases.

  • The tail can be long

    Post-infectious lactose intolerance and irritable bowel symptoms are common - patience and a short low-lactose period help.

How the diagnosis is made

From first symptoms to a confirmed diagnosis.

The steps a UK GP or specialist will normally follow, in order - so you know what to expect and why.

  1. 01

    Assessing

    Focused history

    Recent travel, hiking, wild swimming, unfiltered water, nursery contacts, pets and household illness - all of it matters.

  2. 02

    Assessing

    Symptom pattern

    Watery or fatty, foul-smelling diarrhoea lasting more than a week, with bloating and weight loss, points strongly at giardia.

  3. 03

    Assessing

    Examine and hydrate

    Weight, hydration and abdominal exam - dehydration and significant weight loss change the urgency of investigation.

  4. 04

    Confirming

    Stool testing

    Microscopy for cysts and trophozoites, antigen ELISA and PCR panels - multiple samples improve sensitivity because shedding is intermittent.

  5. 05

    Confirming

    Rule out mimics

    Coeliac disease, inflammatory bowel disease and irritable bowel syndrome can look similar - baseline bloods and coeliac serology are worth doing.

  6. 06

    Escalating

    Specialist tests if refractory

    String test or duodenal aspirate at a specialist centre - reserved for atypical or treatment-resistant disease.

  7. 07

    Escalating

    Refer when needed

    Gastroenterology or tropical medicine input for refractory cases, immunocompromised patients or unusual travel histories.

Typical timeline: from first stool sample to a confirmed diagnosis in days, not weeks.

Symptoms

What giardia actually feels like.

A recognisable mix of diarrhoea, greasy stools, bloating and weight loss - and the features that mean it is time to seek help.

  • Watery diarrhoea

    Often the first symptom - loose, frequent stools that can go on for weeks if untreated.

  • Foul-smelling steatorrhoea

    Pale, greasy, floating stools that are hard to flush - a signal of fat malabsorption from small-bowel infection.

  • Bloating and flatulence

    Excessive wind and a distended abdomen - the classic "sulphurous" giardia burps and gas.

  • Abdominal cramps

    Crampy, upper-abdominal pain that comes and goes with meals - rarely severe on its own.

  • Weight loss and fatigue

    Unintentional weight loss and low energy from malabsorption - striking in a previously well adult.

  • Nausea and poor appetite

    Queasiness and a genuine aversion to food - usually settles once treatment starts.

  • Chronic and relapsing course

    Symptoms that wax and wane for months - sometimes with transient lactose intolerance afterwards.

  • Red flag - failure to thrive in children

    Poor growth, faltering weight or persistent diarrhoea in a child needs prompt paediatric review.

Treatment

How giardia is treated in the UK.

Metronidazole first, tinidazole as a single-dose alternative, and nitazoxanide or paromomycin when they are not suitable - all alongside rehydration and hygiene measures.

  • Metronidazole

    Oral 400 mg three times a day for five to seven days - the UK first-line. Avoid alcohol during and for 48 hours after.

  • Tinidazole

    Oral 2 g as a single dose - a convenient alternative with similar efficacy and the same alcohol advice.

  • Nitazoxanide

    A useful alternative in adults and children, particularly where metronidazole is not tolerated or has failed.

  • Paromomycin

    A non-absorbed aminoglycoside used in pregnancy - less effective but avoids nitroimidazoles in the first trimester.

  • Rehydration and salts

    Oral rehydration solution, small frequent fluids and electrolytes - the backbone of supportive care.

  • Temporary lactose-free diet

    Post-infectious lactase deficiency is common - a short lactose-free period often eases lingering diarrhoea and bloating.

  • Repeat course if relapse

    A second course of a different agent or a longer duration - specialist input if symptoms return after two attempts.

  • Household and partner care

    Selective treatment of symptomatic contacts and strict hand hygiene - decided case by case with specialist advice.

What this guide is based on

The sources behind every claim on this page.

UK national guidance and specialist society standards, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP or infectious diseases team knows your history and can tell you which parts apply to you. If in doubt, get seen.

  • UKHSA. Giardiasis - notifiable disease guidance and outbreak management.

  • NICE CKS. Gastroenteritis - assessment and management, including giardia.

  • BNF. Metronidazole, tinidazole, nitazoxanide and paromomycin monographs.

  • Hospital for Tropical Diseases (UCLH). Guidance for returning traveller diarrhoea and refractory giardiasis.

Red flags

When giardia needs urgent attention.

Most cases are managed in primary care. These are the situations where a specialist opinion or same-day review is needed.

  • Severe dehydration

    Reduced urine output, dizziness on standing, dry mouth or lethargy - needs same-day medical review and often intravenous fluids.

  • Bloody diarrhoea

    Giardia rarely causes blood in the stool - if present, think dysentery, inflammatory bowel disease or E. coli and seek review.

  • Failure to thrive in children

    Poor weight gain, faltering growth or prolonged diarrhoea in a child warrants prompt paediatric assessment.

  • Immunocompromise

    HIV, chemotherapy, transplant or primary immune deficiency - infection can be prolonged and needs specialist input.

  • Pregnancy

    Metronidazole is generally avoided in the first trimester - paromomycin is preferred and specialist advice is sensible.

  • Persistent symptoms after treatment

    Ongoing diarrhoea beyond two to four weeks after a completed course - consider treatment failure, reinfection or a mimic like coeliac disease.

  • Outbreaks and clusters

    Two or more linked cases in a nursery, household or workplace - notify UKHSA and start outbreak measures early.

  • Returning traveller

    A febrile or systemically unwell traveller needs urgent review - not everything that started abroad is giardia.

  • Reactive arthritis or urticaria

    Post-infectious complications are recognised - they need review but usually settle with treatment of the underlying infection.

Living with it

A short illness, with practical steps.

Four things that make the biggest difference day to day - hydration, gentle food, careful hygiene, and preventing the next episode when you travel.

A quiet reminder

The tail can be longer than the illness.

A few weeks of loose stools, bloating or lactose intolerance after successful treatment is normal - not a sign of relapse.

  1. 01 Hydrate

    Small sips, often

    Oral rehydration salts, water and clear soups - little and often beats trying to catch up in one go.

  2. 02 Diet

    Go easy for a fortnight

    Bland, low-fat, low-lactose food while your gut resettles - reintroduce dairy slowly if bloating lingers.

  3. 03 Hygiene

    Break the transmission chain

    Meticulous handwashing after the toilet and before food - stay off work, school or nursery until 48 hours symptom-free.

  4. 04 Prevent

    Filter or boil in the wild

    Treat all surface water when hiking or travelling - filter, boil for one minute, or use iodine or chlorine dioxide.

Frequently asked

Everything we get asked about giardia.

Quick answers on how it spreads, how it is tested for, treatment and getting back to normal life.

  • What is giardia infection?

    Giardiasis is an infection of the small intestine caused by the protozoan Giardia duodenalis (also called G. lamblia or G. intestinalis). It is the most common intestinal parasite in the UK and is picked up by swallowing cysts from contaminated water, food, hands or surfaces.

  • How did I catch it?

    Almost always faecal-oral. The classic routes are unfiltered surface water (a reason it is nicknamed beaver fever among hikers), foreign travel, nurseries and day care, contaminated food, and contact with infected people or animals such as dogs. Symptoms usually appear one to three weeks after exposure, which is why the trigger is often forgotten.

  • How is it diagnosed?

    Stool testing is the mainstay - microscopy for cysts and trophozoites, giardia antigen ELISA, and increasingly stool PCR panels that pick up several pathogens at once. Because shedding is intermittent, sending two or three samples on separate days improves the pick-up rate. A specialist may occasionally arrange a string test or duodenal aspirate for refractory cases.

  • What is the first-line treatment?

    In the UK, oral metronidazole 400 mg three times a day for five to seven days is the standard first-line option. Tinidazole 2 g as a single dose is an equally reasonable alternative. Both work well and both need you to avoid alcohol during treatment and for 48 hours afterwards.

  • What if the diarrhoea persists after treatment?

    A few weeks of bloating, loose stools or lactose intolerance is common after any small-bowel infection and usually settles with a short lactose-free period. If real symptoms persist beyond that, we consider treatment failure, reinfection or a different diagnosis such as coeliac disease or inflammatory bowel disease, and either repeat the course with a different agent or refer to gastroenterology.

  • Can I go back to work or send my child to nursery?

    Stay off work, school or nursery until you have been 48 hours symptom-free, and be extra careful with handwashing for another week or two afterwards. Giardia is a notifiable disease in the UK - your GP or laboratory will inform UKHSA, and for nursery or food-handling cases there are specific exclusion rules to follow.

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